1. Field of the Invention
This invention concerns a method for shortening the distributed queue which exists over a number of nodes connected to a digital transmission line.
2. Description of the Related Art
In "The QPSX MAN", R. M. Newman et al., IEEE Communications Magazine, Vol. 26 (April 1988) No.4, p. 20-28, an arrangement is introduced which digital information is transmitted between several nodes over two non-branching parallel buses. Nodes are situated one after the other and are attached to both buses. Contiguous, address-labelled packets run in one direction only on each of the two buses at the same rate. Every packet contains, in the header part, two bit segments which are necessary to create distributed queues over all nodes. One of these bit segments is used to signify "empty" (E) or "busy" (B). The other bit segment is used to signify "request" (R) or "no request" (N). According to the values of these bits, packets will subsequently be referred to as "empty packets" (E), "information packets" (B), "request packets" (R), and "non-request packets" (N).
The distributed queue enables every node to access the buses at more-or-less any time, which is particularly important when short, urgent messages (e.g. alarms) must be sent. A queue is created when there is a lot of traffic on the bus, by means of a continuously operating protocol. A partial representation (image) of the queue is found in each node. This protocol is described as follows:
In every node, an entry is read into a FIFO (First-In, First-Out) memory whenever a request packet (R) is forwarded, and an entry is read out of the same FIFO memory whenever an empty packet (E) is forwarded in the opposite direction. Through this action, the FIFO memory creates the aforementioned image of the distributed queue. PA1 When a node wants to send information in one direction, it sends a request signal in the opposite direction to that in which it wants to send the information. The node does this by changing the first non-request packet (N) it receives into a request packet (R) and sending this out. It also adds a later-recognisable entry into the FIFO memory. PA1 As soon as the particular entry representing the above request-to-send is read out of the FIFO memory, the node is authorised to send its information. The node does this by filling the next arriving empty packet (E) with its information and sending the packet out as an information packet (B). The next request to send information can then be actioned.
This described protocol operates satisfactorily and balances the access requirements between nodes at all times. A disadvantage, however, is that it is not possible to extend this protocol to genuine ring-structured systems (without special `head-of-bus` units).